Note: All individuals who are diagnosed by the investigator as with splenic dysfunction are eligible for enrollment in the At-risk group. When available, investigator will collect medical documentation for reduced splenic function diagnosed with an appropriate technique in the At-risk subject's medical records. No further assessment will be necessary. A maximum of 35 individuals with sickle-cell disease can be enrolled in the At-risk group. These subjects do not require assessment of the splenic function as sickle-cell disease is invariably associated with severe splenic dysfunction.